Spectroscopy and Photometry of the Double-lined Binary HD 149420

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Abstract

We have used new spectroscopic and photometric observations of HD 149420 to obtain an improved orbital element solution and determine some basic properties of the system. This double-lined binary has an orbital period of 3.39430 days, a circular orbit, and a mass ratio of 0.52. The primary is somewhat evolved, with an A9 IV spectral type, while the secondary is estimated to be an F5: dwarf. The secondary may be synchronously rotating, but the primary is rotating substantially slower than its synchronous velocity. Our photometric observations confirm the light variability and period found by Hipparcos. We determine a photometric period of 1.698 days, essentially half the value of the orbital period, with a peak-to-peak amplitude of 0.028 mag in V. In addition to this ellipsoidal light variation, we also detect more rapid, low-amplitude variability with a period of 0.076082 days and a peak-to-peak V amplitude of 0.008 mag, as well as a possible second short 0.059256 day period with a peak-to-peak amplitude of only 0.005 mag. This indicates that the A9 IV primary in HD 149420 is a pulsating δ Scuti variable. Our binary model that best fits both the radial velocity and photometric observations indicates the possibility of extremely shallow grazing eclipses, a circumstance not unexpected given the large minimum masses from the orbital solution, the evolved nature of the primary, and the short orbital period.

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